Civil and Environmental Engineering


First Advisor

Chung R. Song

Date of this Version

Fall 2018


Bahmyari, H. (2018). "Shear Strength Characteristics and Failure Mechanism of Slopes in Overconsolidated Soils of Nebraska", University of Nebraska-Lincoln, Lincoln. NE.


A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Civil Engineering, Under the Supervision of Professor Chung R. Song. Lincoln, Nebraska: Fall 2018.

Copyright (c) 2018 Hossein Bahmyari


Geotechnical engineers deal with some challenging problems when encountering long-term slope stability in overconsolidated clays and clayey shales. One of these problems is determining a suitable method to estimate the reliable shear strength of overconsolidated clayey soils for long-term slope stability. According to Eversol (2013), one half of the counties in Nebraska have experienced slope failure in overconsolidated soils, especially in north and east Nebraska. In this research, shear strength characteristics of overconsolidated clayey soils of Nebraska is investigated in regard to long-term stability of slopes, and the failure mechanism in such soils is examined.

Undisturbed samples from boring logs from two different failure locations were taken. The research involved laboratory testing to compare and investigate the shear strength parameters of overconsolidated soils in undrained and drained conditions at high and low normal effective stress levels. Some issues that are addressed include: (i) the shear strength reduction due to low effective stress in adrained condition, (ii) the swelling behavior during consolidation and shearing stages of saturated overconsolidated soils in low effective confining stress, (iii) the suitable shear strength that helps to design/repair slopes in overconsolidated soils of Nebraska.

One major finding of this research is that one of the main reasons for slope instability in overconsolidated clayey soils and shales in Nebraska is shear strength reduction. This strength reduction, which usually causes a progressive failure in overconsolidated soils in Nebraska, is due to swelling and softening; (2) the presence of expansive clay minerals in overconsolidated soils induced additional swelling in some areas reduces the shear strength of soils dramatically, especially at shallow depths; and (3) the laboratory results showed that the fully softened shear strength from a consolidated drained triaxial test may present a suitable shear strength for long-term slope stability in overconsolidated soils in Nebraska.

Advisor: Chung R. Song